1. Field of the Invention
The present invention relates to the field of spin coating, and in particular, the spin coating of chemical having uniform thickness onto a substrate.
2. Description of the Related Art
In a variety of fields, a thin film or coating of material on a substrate, such as data recording media or semiconductor wafers, is desired. In the field of fabricating semiconductor devices, for example, a thin layer of photoresist is commonly applied to a semiconductor wafer using a spin coating apparatus.
A typical spin coating apparatus comprises a chemical supply source, a coating element for introducing the chemical onto the substrate or wafer, and an exhaust element for removing excess chemical and contaminated air. Various properties and adaptations of spin coating apparatus are described in, for example, U.S. Pat. No. 6,165,267 issued Dec. 26, 2000 to Torczynski; U.S. Pat. No. 5,562,772 issued Oct. 8, 1996 to Neoh; U.S. Pat. No. 5,449,405 issued Sep. 12, 1995 to Cardinali et al.; U.S. Pat. No. 5,234,499 issued Aug. 10, 1993 to Sasaki et al.; U.S. Pat. No. 4,800,836 issued Jan. 31, 1989 to Yamamoto et al.; and U.S. Pat. No. 4,794,021 issued Dec. 27, 1988 to Potter, which are hereby incorporated herein by reference.
In typical prior art apparatus, the chemical supply source provides the desired chemical, commonly dissolved in a volatile solvent, to the coating element which includes a chemical dispensing means positioned above the wafer. The wafer, which is generally mounted on a rotatable chuck positioned within a coating bowl, receives the chemical from the chemical dispensing means, typically through one or more nozzles. The chemical is dispensed at or near the center of the wafer. The wafer on the chuck is then spun at high speed, at a particular rotation rate, using a motor assembly. This causes the chemical solution on the wafer to flow toward the edge of the wafer. As the solvent evaporates, the chemical solution becomes more and more viscous until the solution becomes so viscous it stops flowing. The remaining solvent eventually evaporates, leaving a thin film or coating on the wafer. The thickness of the chemical film on the wafer depends on a number of factors including, for example, the rate of rotation of the chuck and the rate at which the solvent evaporates.
It is known that if the solvent evaporates from a thinning film in a uniform manner, then the thickness of the resulting dry film will be of uniform thickness. Conversely, non-uniform solvent evaporation will result in a non-uniform film thickness. Solvent evaporation non-uniformity can be caused by non-uniformity in the gas flow over the spinning wafer. Because of the rotation, non-uniformities in the film thickness will tend to occur primarily in the radial direction rather than the circumferential direction.
To remove the evaporated solvent and excess chemical, a vacuum pump may be attached to the exhaust port to draw air into the coating bowl and over the exposed wafer surface, and then into the exhaust port and finally out of the spin coating apparatus into the vacuum pump system. In some cases, airflow is directed along the wafer underside into the exhaust port to minimize contamination of the wafer underside.
In a conventional spin coating apparatus, two main parameters govern the gas flow: the vacuum pumping rate and the rotation rate. Typical values used in the art are pumping rates of 250-1000 liters per minute (1 pm) and rotation rates of 0-5000 revolutions per minute (rpm). Because of the effect of viscosity, a swirling component of velocity is imparted by the substrate to the adjacent gas. This gas is centrifuged radially outward toward the outer edge of the substrate. The continuing removal of gas adjacent to the substrate in the radial direction draws air from the ambient supply above the substrate toward the surface to replace the displaced gas. At the same time, the vacuum pump system is removing gas via the exhaust port. Therefore, the airflow over the substrate is governed by the complex interaction of airflow due to the substrate pumping air by centrifuge action, airflow due to the vacuum pumping system, and the aerodynamic geometry of the spin coating apparatus.
In a conventional spin coating apparatus, the complex interaction of flows and geometry is known to result in the formation of wafer eddies. These eddies return a portion of the centrifuged gas back to the wafer surface. The changes in airflow above the wafer where the eddies are located causes the solvent to evaporate at a different rate than it would in the absence of such eddies. This local non-uniform rate of solvent evaporation results in non-uniform chemical film thickness.
The spin coating process is but one of many steps in the manufacturing of semiconductor devices. It is known in the art that even small thickness variations of the spun-on chemical film, on the order of {fraction (1/10)}th of one percent of the mean film thickness, can result in lower yields of suitable semiconductor devices. As the diameter of the semiconductor wafer increases, the uniformity of the thickness of the film becomes increasingly important.
It is common in the art to use complicated empirical strategies to achieve the required uniformity of film thickness. In some cases, for example, the dispensed chemical is heated slightly and the nozzle is moved over the wafer in a complicated trajectory during dispensing. Such devices can add considerable complexity and initial cost, and the heating and nozzle trajectory must be changed every time a new chemical is used or the wafer size is changed.
A variety of other devices have been designed in an attempt to improve the uniformity of film thickness. Many attempts have been made to slow the evaporation rate of the chemical solution on the wafer surface and to prevent local air disturbances at the wafer surface. In particular, multiple attempts to minimize or eliminate the surface effects from the ambient air by imposing a barrier, serving as a means to isolate the local air over the wafer, have been attempted. See, e.g., U.S. Pat. No. 6,261,635 issued Jul. 17, 2001 to Shirley; U.S. Pat. No. 6,238,735 issued May 29, 2001 to Mundal et al.; U.S. Pat. No. 5,472,502 issued Dec. 5, 1995 to Batchelder; U.S. Pat. No. 4,800,836 issued Jan. 31, 1989 to Yamamoto et al., and U.S. Pat. No. 4,587,139 issued May 6, 1986 to Hagan et al. These attempts, however, introduce additional complications by impeding the airflow above the wafer and often add considerable cost and complexity.
Significant surface effects that contribute to the non-uniformity of film thickness, known as Ekman spirals, are known in the art (see, e.g., 77 J. Appl. Phys. 2297, The Connection Between-Hydrodynamic Stability of Gas Flow in Spin Coating and Coated Film Uniformity, 1995). These spirals include both Type-I or xe2x80x9cstationaryxe2x80x9d spirals, which rotate at the same speed as the wafer and thus do not move with respect to the wafer, and Type-II or xe2x80x9ctravelingxe2x80x9d spirals, which do not rotate with the wafers and therefore move at considerable velocity with respect to the wafer surface. It is highly desirable to interfere with the formation of these Ekman spirals.
A second concern is contamination of the wafer surface by material such as droplets of chemical flung off the wafer during spinning. Airflow as described above can transport much of the excess chemical and solvent to the exhaust port. With conventional spin coating apparatus, high airflow rates can be required to prevent unacceptable wafer contamination. However, higher airflow generally means a higher risk of turbulence and eddy formation at the wafer surface. This can result in local variations of the solvent drying rate, and therefore a higher risk of undesirable thickness variations of the chemical film.
A third concern is contamination of the room environment by material such as volatile solvent or excess chemical. As in the case of wafer contamination, higher airflow rates can be required to prevent unacceptable room environment contamination. However, as already noted, higher airflow generally means a higher risk of turbulence and eddy formation above the wafer, which means a higher risk of undesirable thickness variations of the applied chemical. Also, since most spin coaters are located in special dust-free clean rooms, excessive room contamination results in higher operating costs. While some devices have utilized various forms of coverings, and even sealed coating bowls, these devices introduce added cost and complexity and frequently introduce new airflow-related problems further degrading the chemical layer thickness uniformity.
Accordingly, there is a need for an improved spin coating apparatus of economical design that minimizes thickness variations of the coated chemical and at the same time minimizes wafer and room contamination without introducing additional airflow-related complications at or near the wafer surface.
To achieve these and other objects, the present invention provides a spin coating apparatus and process for applying a coating of uniform thickness to a substrate. In particular, the spin coating apparatus utilizes a perforated sheet of thin but rigid material, such as metal, positioned just above the wafer""s top surface. The perforated sheet extends in a radial direction and may be attached to the coating bowl at the sheet edges. The chemical mixture, typically consisting of photoresist, is dispensed onto the wafer through an opening in the perforated sheet near the center using a conventional nozzle dispensing system. Air is drawn through the perforated sheet and directed over the wafer to the exhaust. Increasing the number of perforations or the size of the perforations, or both, will result in increased airflow, which will result in faster solvent evaporation rates. As the solvent evaporates from the chemical mixture, the viscosity of the mixture becomes larger and larger until the chemical mixture ceases to flow. Hence the local thickness of the spread film becomes fixed. The size and number of perforations can be adjusted, by one skilled in the art, in a radial direction from the center to the edge so as to control the local rate of solvent evaporation from the chemical film in a radial direction. This, in turn, allows some control of the radial thickness variations of the applied chemical.
By experiment, an optimum height of the perforated sheet above the wafer surface is determined. At the optimum height, airflow instabilities above the wafer are prevented from forming. Airflow instabilities can change the local evaporation rate of the solvent, which, in turn, can adversely affect the thickness uniformity of the deposited chemical. Optimum height is defined as the height that minimizes film thickness non-uniformity.
The perforated sheet also acts as a partial physical barrier that reduces the amount of wafer surface and room contamination from excess chemical and solvent that re-circulates after it is thrown off the wafer during spinning thereby impacting into the sidewalls of the chemical bowl.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.